I've been using the nitrocellulose-aerosols to paint the guitar, the steps I've done are:
Sanded - except the front as it's veneer
Applied sprays of sanding sealer
Applied several sprays of white primer - leaving about 30 mins in between
Sprayed the front and sides of the body green, and the back and the neck black gloss - again about 30 mins in between.
At this point the black gloss was going on very evenly and smoothly, but the green took many more coats as the first few were streaky. I should point out that I didn't really do any light sanding in between coats.
I also bought some clear gloss from the site to add extra layers of protection.
When I started to apply the clear gloss the next day, the body and the neck became very streaky, almost like it was reacting. I now realise that I probably need to let the paint layers cure for at least a week before applying the clear gloss.
So I sanded everything back down and re-applied the colours, and it's back to looking good. So I just wanted to ask how long I should leave it before applying clear gloss, as I don't want the same thing to happen again. All paints and gloss were bought from the same site, unfortunately their instructions are a little lacking and they don't respond to emails asking for advice.
Do I need to sand between gloss coats? Or could that remove the paint?
Nitro is very demanding of temperature and relative humidity. Too cold or too humid and moisture gets trapped under the finish and it goes cloudy in patches. It's been marginal outdoor spraying weather for a lot of the summer here.
I've mainly used Rothko and Frost nitro, and some Northwest guitars nitro, but as all the guitar nitro suppliers seem to be based in Manchester, I suspect they all have a common supplier. I've used some rattle sprays for convenience, but mainly the stuff in bottles or tins for use with my mini spray gun. With that, you can add different thinners for different situations. You can add retarder for when you want the paint to be a bit thinner and dry more slowly so it levels out better, and also anti-bloom agents, which allow you to work in more humid or colder environments without the fogging.
But without photos, it's hard to tell exactly what's happened to your paint.
What you should be doing between different layers of paint i.e. sanding sealer, undercoat, colour coat and clear, is sanding back to get the smoothest surface you can get if you want a nice shiny polished finish.
So after sanding sealer - sand
After the undercoat - sand
After the colour coat - sand
After the clear coat - sand and then polish
The less you sand the coats, the more you need to sand the final coat to get it level with no dips, peaks or orange peel. And the more you sand that, the more risk there is of sanding through to the previous coat.
You don't need to wait before applying clear nitro over gloss nitro paint. It won't react because of that. But you do need to wait a few days after spraying for the paint to harden enough to sand it properly.
Ambient conditions will determine how quickly you can apply spray coats. In ideal conditions - say 20°C-24°C and low humidity, you can get away with 30 minutes between coats. But it really is better if you an wait longer. No more than three coats a day and maybe 4 hours between coats, so a morning lunchtime and late afternoon spray is ideal. This allows time for the majority of the solvent and possible moisture in the coat to evaporate.
Acrylic sprays (like car paint) require re-coating within 30 minutes or otherwise after 24 hours and sanding, as the acrylic needs either a still slightly wet surface to adhere to or else a rough one to key to. The organic solvent in nitro (mainly acetone) will melt the surface of any nitro under it (it could be years old) and adhere to that. The less solvent that gets trapped under successive layers of nitro, the less time it will take to 'gas-off' and cure enough to be sanded and take a polish.
So whilst a light coat of nitro on a good spraying day can be coated again in 30 minutes, it's best to wait longer if you can. Even an hour is better than 30 minutes.
In terms of coat numbers, it depends on whether you are applying clear coat or not.
Sanding sealer - enough coats to fill in any gaps. On an ES-1, provided its basswood veneer, you may not need it at all, but 1 to 2 coats should be OK. On mahogany or ash, you'd need a lot more (as well as some grain filler). But you want to sand this back flat (hence the term 'sanding sealer'). If you can get the surface of the guitar really flat here, then it makes life a lot easier later on. If it's a rough surface, then I'd start with P240 and move up to P600. If smooth, maybe P400 and then P600. But nothing too coarse and as you will be adding extra layers on, you don't need to go too fine.
Undercoat - not always required if you have sanding sealer and you don't need a white finish to help with a light-coloured colour coat. But 3 coats max and again, then sand back flat. This is normally quite powdery, so you want a minimal layer left.
Again, as smooth (in trems of no dips or bumps or orange peel) a surface as you can get. So leave for a couple of days after spraying before sanding.
Colour coat - maybe 3 coats if being clear coated, 6 coats (3 per day) if not. Sand smooth after a couple of days up with P600 if being clear coated. If this is the final coat (often the case on non-bound bodies), then wait at least two weeks (a month is better) before sanding and polishing.
Clear coat - I'd suggest 6 coats, and try and apply more on edges and ridges as sandpaper will apply more pressure and remove more finish on these. Again wait at least two weeks (a month is better) before final sanding and polishing.
I'll often give an initial final sand with P400 after a few days, as the rougher surface gives greater surface area for the solvent gases to escape through (well that's the theory anyway). It's the polishing that needs the nitro to be fairly well cured (apparently it can never really be considered as fully cured until the point it starts to craze with age) otherwise if won't take a high shine.
I prefer wet sanding, though it can cause issues with water getting into the edges of pot and switch holes and expanding the wood, but some will dry sand to avoid this. Wet sanding generally avoids the paint particles clogging the sandpaper, as these can themselves cause scratches bigger than the sandpaper grit ones, so always keep an eye on the paper and replace it if too clogged.
For final sanding, I'd probably start with P600 and go up through the grits. Depending on what you have to polish the finish with you might stop at P1500, but it doesn't harm if you carry on up to P2500, as it's often easier to do this than spend more time polishing by hand.
For 'polishing', you need a rubbing/cutting compound. Car products work well here. I use what I'd class as 'medium', 'fine' and 'very fine' compounds typically T-cut, then Meguiars Scratch-X 2.0, and then MicroMesh Micro-Gloss liquid abrasive.
Ideally you'd a buffing wheel and buffing compounds, but unless you have a buffing wheel, then either polishing by hand using Microfibre polishing cloths and the compounds, or polishing foams on a battery drill (mains-drills rotate too fast and build-up too much heat) will give you a good shiny finish.
Heat is the enemy and softens the surface, so don't concentrate on a small area, but keep moving the cloth around. Even rubbing by hand can create a lot of friction.
For cheaper clear nitro in the future can I suggest this company:
They aren't great for nitro colours. but I recently bought that pack of 6 clear cans (I was doing a headstock and didn't want the hassle of getting the spray gun out and having to clean it) as it was only twice as much as a can of clear from R&F once postage was added - and that Nugane price includes postage (and they included a free pack of fruit gums as a gift). It sprayed really well and produced a very flat gloss finish, probably better than the R&F or NWguitars clear. Only 'downside' is that it only has a round spray nozzle, not a directional fan one, but I didn't find that an issue.
I've attached some pictures, one marked 'Streaky' is after I finished colouring and during the clear sprays.
The one marked 'Finished' is after I then sanded back down and re-applied a few coats of colour. As it's my first one, I'm not looking for perfection so if there are a few lumps and bumps on the body I'm ok with it, the neck was sanded a few more times between steps and I'm happy with the smoothness of that.
I finished painting the neck over a week ago, and finished the body about 2 days ago.
I have some P1500 and P2000 grit sandpaper. Given how the weather is in the UK currently (approx 26c) would now be a good time to start applying the clear gloss to the neck?
Never knew the world of paints was so complicated!
The streaking is weird, though it may be down to the spray pattern you made. Could well be down to moisture bubbles/pockets, though on mine they were a bit more random.
It could also be dry overspray; paint that didn't go on wet but droplets which dried enough on their way from the can to stick but not merge with the rest of the paint. Sometimes down to heat, sometimes having the can too far away from the surface. This can be simply sprayed over with wet nitro as it will melt and be absorbed into the general nitro mass.
Yes, up to 28°C is probably OK without some retarder (the cans normally have a bit of retarder and anti-bloom product, but there's nothing like being able to regulate it yourself to match the conditions), but keep it out of direct sunlight as even with a light colour, the surface can become really hot and the surface of the nitro dries instantly, trapping solvent under the surface which can form bubbles. So, definitely work in the shade when it's warm.
Also let it dry in the shade, at least for the first hour or so. I've had bubbles form in clear after spraying it in too hot conditions and letting it dry in my old dark-coloured spray/toilet tent (my new one is silver-white and far more reflective), where it was well above ambient. If you get those in clear, you need to sand back as you can't spray your way out of it. I tried and failed.
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